As from today, my monitor started showing this menu that disappears and reappears every few seconds. I also had to change picture size to screenfit as 16:9 would no longer fit as it used to. I tested monitor on 2 different PCs and both had same issue with menu and resolution.
Model Code: LT24A350EW/EN Software Version: T-MSTMDEUC-1010.0
I've been using it for some years now and don't recall ever having this issue before.
Edit: it also seems to be automatically switching back to 16:9 screen size.